1. Field of the invention
The present invention relates to an apparatus and method for operating a system comprising two or more fluid treatment units connected in a parallel arrangement. More particularly, the present invention relates to a method for choosing when to regenerate a unit and which unit to regenerate and an apparatus for implementing the same.
2. Description of Related Art
In many applications, it is necessary or desirable to treat a fluid such as water prior to its use. Such treatments include but are not limited to softening of water, deionization of water, or filtering to remove solid matter from a fluid. To perform the desired treatment, the untreated fluid flows through a fluid treatment unit, which comprises one or more treatment stages, and flows out as treated fluid.
A plurality of fluid treatment units may also be connected in parallel. In this configuration, the stream of untreated fluid diverges into a plurality of streams with each separate stream of untreated fluid flowing into a separate fluid treatment unit for treatment and flowing out as a separate stream of treated fluid before being recombined into a common stream of treated fluid for use.
In the case of water softening, the fluid treatment unit is generally a pressurized tank containing an ion-exchange resin. As the water passes through the resin, objectionable ions present in the water, such as calcium, are exchanged for less objectionable ions present in the resin, such as sodium. As the resin in a particular unit is used to treat more and more water, it gradually becomes depleted of its exchange ions. The resins thus generally have a capacity to treat only a certain volume of water, and, if this volume is exceeded, the water flowing through thereafter will not be adequately treated.
Once they become depleted, normally the commercially available resins may be regenerated in order to be able to treat more water. Regeneration of these resins typically involves exposing them to a suitable solution, such as a brine, which contains the desired exchange ion. For water softening applications, the brine is typically made by dissolving a quantity of salt in water. The initial capacity to treat water of the newly regenerated resin can be determined based on the quantity of salt used in the regeneration process and the nature and volume of the resin.
Similarly, a filter used to remove solid matter will eventually become clogged with material and ineffective after a certain amount of use. Different filters thus have different capacities in terms of the volume of water they can effectively filter. Filters can often be regenerated by backflushing or similar processes.
During the regeneration process of a fluid treatment unit, however, the unit will not be available for treatment of the fluid. Therefore, in order to prevent interruption of service, several units are often connected in parallel. The advantage of this configuration is that when one unit is taken out of service for regeneration or for some other reason, fluid can still pass through the units that remain in service, thus allowing the fluid treatment process to continue despite the ongoing need to regenerate units.
Several different methods are known in the art for automatically regenerating fluid treatment units connected in a parallel configuration so as to minimize interruption of service. The simplest method is to regenerate a unit after a preset time has elapsed. One of the disadvantages of this method is that the actual remaining capacity of the unit after the preset time has elapsed is not taken into account. For example, if water demand has been low, then the unit may be regenerated even though it has much remaining capacity, resulting in a waste of regeneration salt. On the other hand, if water demand has been exceptionally high, then the unit may have become exhausted before the time period has elapsed for it to be regenerated. Inadequately treated water would have flown out of the system as a result.
Chili et al., U.S. Pat. No. 5,073,255 discloses the use of a bridge sensor to measure the conductivity of the ion exchange resin and to thereby regenerate the resin when the conductivity reaches a certain value. This method, however, is generally useful only for ion exchange systems. It could not be easily applied to systems which only filter the water.
Brane et al., U.S. Pat. No. 5,300,230 discloses a method whereby a regeneration is initiated whenever a certain volume of water, V.sub.s, has been treated by any one of the units, where V.sub.s is the capacity of the smallest unit divided by the total number of units. The units are then regenerated sequentially. This method may require needlessly frequent regenerations if one of the units has a much lower capacity than the others. In addition, merely regenerating the next unit in a sequence does not take into account which unit may need regeneration the most. For example the "next" unit in the sequence may have a great deal of remaining capacity.